If a rubber ligature should come off, you may be able to put it back in place using sterile tweezers. If a wire ligature comes loose, simply remove it with sterile tweezers. If the wire ligature is sticking out into the lip but is not loose, it may be bent back down with a Q-tip or pencil eraser to eliminate the irritation.
Of course, when one ligature pops off or breaks, others may follow. Be sure to examine all ligatures before sending the student back to class. Missing or broken ligatures should be brought to the attention of the student's parents, who should then inform the orthodontist.